1. Field of the Invention
This invention relates to elastomeric bushings and, more particularly, to bushings which are adjustable to permit their use in openings of varying dimensions. In another aspect, the invention relates to suspension systems for vehicles with improved bushings.
2. Description of Related Art
Elastomeric bushings have long been used in mounting one part to another while permitting limited articulation between the two parts. Such elastomeric bushings typically comprise a metal outer tube, a metal inner tube, and an elastomeric insert mounted between the inner and outer tubes. The elastomeric insert is slightly compressed. Such a bushing configuration is disclosed in U.S. Pat. No. 2,550,564 to Hutton, issued Apr. 24, 1951, and U.S. Pat. No. 2,110,783 to Welker, issued Mar. 8, 1938.
In a typical installation, the bushing is used to connect a first member with a second member in an articulatable connection. The outer tube of the bushing is usually received within an aperture in the first member and the second member is received within or connected to the inner tube. The outer tube is securely fixed to the first member and the second member is securely fixed to the inner tube. The intermediate elastomeric portion serves to permit limited articulation of the first member relative to the second. A typical application includes the bushed connection of an axle to a trailing arm in a trailing arm air spring suspension for a vehicle.
In a typical elastomeric construction, the elastomeric insert is connected to the outer and inner tubes by either bonding the elastomeric insert to the tubes or by compressing the elastomeric insert between the tubes. The elastomeric insert functions in a manner similar to a spring. As the inner metal tube articulates or rotates with respect to the outer metal tube, the elastomeric insert is extended and stretched. The resilient nature of the elastomeric insert resists the articulation or rotation and tends to urge the inner and outer metal tubes to their original relative positions. The amount of articulation between the inner metal tube and the outer metal tube is limited by the thickness or apparent fiber length of the elastomeric insert. If the articulation is great enough that the elastomeric insert is stretched beyond its limit, the bond between the elastomeric insert and the inner and outer metal tubes will fail and the elastomeric bushing will slip with respect to either or both of the inner and outer metal tubes. A typical elastomeric bushing having a slightly compressed elastomeric element can articulate approximately 20 degrees before the elastomeric insert begins slipping with respect to the inner and outer metal tubes.
In an application where the elastomeric bushing connects a vehicle trailing arm to the vehicle, a much greater degree of articulation is needed between the inner and outer metal tubes than that because of the much larger forces and increased range of movement. A slightly compressed elastomeric bushing is inadequate. Suitable elastomeric bushings having a greater degree of articulation are manufactured by the Lord Corporation of Erie, Pa. and Clevite Products of Milan, Ohio. These elastomeric bushings achieve much greater articulation by highly compressing the elastomeric insert between the inner and outer metal tubes.
In an uncompressed elastomeric bushing, the amount of relative articulation between the inner tube and the outer tube before the elastomeric insert begins to slip is a function of the resiliency of the elastomeric insert. Once the elastomeric insert has stretched to its resilient limit, any continued articulation will cause the elastomeric insert to slip. It is helpful to think of the elastomeric insert being comprised of fibers extending radially outwardly. The apparent fiber length of the fibers for an uncompressed elastomeric insert is the amount the elastomer will stretch. Thus, the apparent fiber length is a function of the resiliency of the elastomeric insert, making the relative articulation a function of the apparent fiber length. In other words, the elastomeric insert will not begin to slip until the fibers are stretched to their resilient limit or apparent fiber length.
In a compressed elastomeric bushing, the apparent fiber length is a function of the amount the elastomeric insert is compressed (the difference between the uncompressed thickness and the compressed thickness of the elastomeric insert) and the resiliency of the elastomeric insert. The relative articulation of the elastomeric bushing initially extends the fibers of the elastomeric insert to their uncompressed length before the fibers begin to stretch to their resilient limit. The compressed elastomeric insert will not begin to slip until the fibers are extended the amount the elastomeric insert is compressed and the amount the fibers will stretch. Therefore, the greater the elastomeric insert is compressed, the greater will be the relative articulation of the elastomeric bushing before the elastomeric insert will begin to slip and the greater will be the apparent fiber length. However, because of the highly compressed state of the elastomeric insert and the need to keep the elastomeric insert compressed to obtain the benefits of a greater apparent fiber length, such elastomeric bushings must be constructed to withstand the large expansion forces associated with the compressed elastomeric insert.
In using such a bushing in a trailing arm suspension, the first member typically comprises a trailing arm. Usually, such trailing arms are formed by casting, fabricating or drop-forging processes. The aperture that results from such manufacturing operations typically has two distinct problems which may drastically affect the mounting of the bushing in the aperture. First, the average diameter of the aperture will not achieve a range of close tolerances. For example, the best available tolerance of the diameter of an aperture in a drop-forged trailing arm is approximately .+-.0.015 inches. This creates significant problems in mounting a bushing within the aperture. If the diameter of the aperture is less than the diameter of the outer tube, the bushing cannot be received within the aperture. If the diameter of the aperture significantly exceeds the diameter of the outer tube, the outer tube will be loosely mounted in the aperture, increasing wear on the bushing, or requiring a welding operation to securely fix the outer tube within the over-sized aperture.
The second problem which often results from a drop-forged or fabricated control arm is that the aperture can be out-of-round at any point along the axis of the aperture. The aperture may be elliptically shaped at various points along the length thereof. If this elliptical shape exists, the outer tube may or may not be mountable within the aperture. For example, if the shorter diameter of the elliptical opening is less than the diameter of the outer tube, but the longer diameter of the elliptical shape is greater than the diameter of the outer tube then the outer tube can still be received within the aperture if the overall circumference of the elliptical opening of the aperture is greater than the circumference of the outer tube. The outer tube will flex somewhat, to assume the elliptical shape and accommodate the shorter diameter. However, if the overall circumference of the elliptical opening does not exceed the circumference of the outer tube, then the outer tube cannot be received in the aperture. Therefore, it is seen that the outer tube can accommodate some variations in the geometry of the aperture, however, there is a limit to the variations which the outer tube can withstand.
An additional problem is that the elastomeric bushings are typically mounted within the aperture by applying a large axial force to the elastomeric bushing, driving the elastomeric bushing into the aperture. The outer tube of the elastomeric bushing must be capable of withstanding the large shear forces between the inner surface and edge of the aperture created by the large axial force.
One solution for overcoming the tolerance and the out-of-round problems is to machine or otherwise finish the aperture to a suitable shape and dimension for receiving the bushing. Unfortunately, such operations are costly, time consuming and labor intensive and therefore not preferred.
Another solution to accommodate dimensional and geometrical differences in the aperture is to eliminate the outer tube from the bushing and press-fit only the inner tube and elastomeric portion into the aperture. The elastomeric portion does not have the same tolerance limitations as the metal outer tubing and can accommodate the geometrical and dimensional variations in the aperture. Unfortunately, this structure results in increased wear on the elastomeric portion, thereby shortening the life span of the bushing. The life span of the bushing is effectively cut in half by eliminating the outer sleeve and utilizing only the elastomeric portion and inner tube, therefore, making this solution decidedly unacceptable.
Still another solution is to provide the bushing with a variable outer diameter, such as the bushing disclosed in the French patent to Menard, 680,434, issued Apr. 29, 1930. Menard is not suitable for use in a vehicle suspension comprising a trailing arm because the outer tube of Menard is not strong enough to restrain the expansion of the highly compressed elastomeric insert, negating the advantage of an increased apparent fiber length by compressing the elastomeric insert. Moreover, the outer tube of Menard is not capable of withstanding the large shear forces imparted to the elastomeric bushing during insertion.